1. Field of the Invention
This invention relates to inspection systems and, more particularly, to systems and methods for simultaneously inspecting a specimen with two distinct inspection channels.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a substrate using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during the fabrication of semiconductor devices to detect defects on wafers, thereby promoting higher yield and higher profits. Inspection has always played an important role in semiconductor fabrication. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices. For instance, detecting defects of decreasing size has become increasingly necessary, since even relatively small defects can cause unwanted aberrations in the semiconductor device.
Many different types of inspection tools have been developed for the inspection of semiconductor wafers, including optical and E-beam systems. Most optical inspection systems are characterized as either bright-field (BF) or dark-field (DF) systems. These inspection systems generally differ in the manner in which light is directed to, and collected from, the surface of a specimen. For example, bright-field inspection systems direct light to a specimen at a particular angle (e.g., normal to the surface of the specimen) and measure the amount of light reflected from the surface of the specimen at a similar angle. Dark-field inspection systems, on the other hand, detect the amount of light that is scattered from the surface of a specimen when light is supplied to the specimen, typically at an oblique angle of incidence. The collection optics used in dark-field systems are typically positioned out of the path of the reflected light so that only scattered light is collected.
The type of inspection tool used for inspecting a particular semiconductor wafer is generally chosen based on characteristics of the wafer, as well as characteristics of the defects of interest. For example, BF inspection systems are configured for detecting defects that primarily reflect light, such as pattern detects. In BF inspection systems, high-resolution imaging optics are combined with a small pixel size to provide images with relatively high spatial resolution (e.g., between about 100 nm and 500 nm). This makes BF imaging particularly useful for detecting and classifying defects whose sizes are on the order of the design rule. However, BF inspection systems tend to be slower than DF systems because of their high spatial resolution.
DF inspection systems are configured for detecting defects that primarily scatter light, such as particles. In dark-field inspection systems, relatively flat areas on the specimen scatter very little light back to the detector, resulting in a predominantly dark image. Surface features or objects protruding above the surface of the specimen scatter light toward the detector to produce light areas in an otherwise dark image. Dark-field inspection systems, therefore, produce dark images except where circuit features, particles or other irregularities exist. In some cases, Fourier filtering may be used in a dark-field system to enhance signal to noise ratios by filtering out the repetitive patterns of light produced by circuit features on the specimen.
Dark-field systems typically provide much higher throughput than bright-field systems. For example, dark-field systems provide a larger pixel-to-defect ratio, permitting faster inspections for a given defect size and pixel rate. In one example, the spatial resolution of a dark-field inspection system may range between about 500 nm and about 2.0 μm.
Typically, no one optical inspection system can detect all defects. As noted above, dark-field systems are generally configured for detecting defects that scatter light, while bright-field systems detect defects that reflect light. Although one-dimensional defect detection algorithms can be separately applied to dark-field and bright-field data to detect both types of defects (within the constraints set by noise sources that limit sensitivity), there are some defects which can only be detected in a two-dimensional decision space.
To illustrate this concept, a two-dimensional histogram of bright-field difference vs. dark-field difference 400 is shown in FIG. 7. The main cloud of data points 410 represents normal pixels, or pixels where no defects are present. In some cases, bright-field defects and dark-field defects can be detected using one-dimensional algorithms, if the light collected from the defects exceeds the bright-field and dark-field noise floors 420 and 440, respectively. For example, satellite cloud 430 represents a defect that may be detected with a one-dimensional algorithm operating on a bright-field image. Likewise, satellite cloud 450 represents a defect that may be detected with a one-dimensional algorithm operating on a dark-field image. However, satellite cloud 460 represents a defect that is below the noise floors of the bright-field and dark-field systems, and therefore, cannot be detected with a one-dimensional algorithm operating on either the bright-field image or the dark-field image alone. Instead, satellite cloud 460 can only be detected with a two-dimensional algorithm operating on the combined results of bright-field and dark-field imaging.
Some prior art inspection systems have tried to increase the range of detectable defects by combining bright-field and dark-field imaging. In one prior art system, bright-field imaging is performed on a wafer during a first inspection run. After results of the first run are processed to determine the bright-field defects, the wafer is illuminated with dark-field illumination in a second inspection run to generate a list of dark-field defects. The problem with this system is that throughput, or the time needed to process a single wafer, is poor and it does not have the capability of using the combined results from the bright-field and dark-field images before the results are processed to determine defects. Instead, the defect detection algorithm used in the prior art system sets thresholds above which a feature is considered a defect, and only passes bright-field and dark-field defects which surpass these thresholds. The prior art system is, therefore, unable to detect defects that fall below the thresholds (or noise floors) individually set for the bright-field and dark-field channels.
In an attempt to solve the above-mentioned problems, a few prior art inspection systems have been designed to provide simultaneous bright-field and dark-field illumination to the wafer. These systems use a single light source (usually a monochromatic or narrowband source, such as a laser) to provide bright-field and dark-field illumination to the wafer. Bright-field and dark-field detectors are positioned within the system, so that reflected light is detected by the bright-field detector and scattered light is detected by the dark-field detector. The information from both detectors is supplied to a defect detection algorithm to determine the location of defects on the wafer.
Unfortunately, currently available systems capable of providing simultaneous bright-field and dark-field imaging have their own set of disadvantages. In the prior art system described above, for example, throughput is improved by providing simultaneous bright-field and dark-field inspection. However, inspection is performed by scanning a spot with a relatively large pixel size across the surface of the wafer. Although high throughput is achieved, the large pixel size significantly decreases the resolution of the bright-field image. In addition, the prior art system uses a single light source for both bright-field and dark-field illumination. This limits the flexibility to independently optimize the characteristics of the bright-field and dark-field illumination. Furthermore, the single light source typically comprises a monochromatic or narrowband source, such as a laser. However, narrowband light sources introduce contrast variations and coherent noise into the bright-field image, which further reduces bright-field sensitivity and resolution.
A need remains for an improved wafer inspection system, which provides simultaneous bright-field and dark-field imaging, while overcoming the disadvantages and limitations of currently available systems.